This invention concerns a device and a method for ion beam etching for producing an etched surface on a semiconductor or an insulant.
Ion beam etching calls for good spatial control of the etching zones on the etched surface. A mask comprising several holes can be used for this purpose. These holes can be several microns in size and enable optimum etching accuracy, in the order of a fraction of a micron, if not smaller. A shortcoming of this technology lies in the fact that it involves realising a mask for each etching pattern.
Another method consists of direct control of the relative position of the incident beam on the etched surface. This control is for example carried out using electric and/or magnetic guiding means and by cooling the beam down, i.e. reduction of the seed component perpendicular to the propagation direction of the beam. The accuracy of this method is further enhanced while associating the direct control with the usage of micro or nano holes. For etching, the semiconductor or the insulant is moved under the beam. This technique improves the accuracy significantly, which reaches a few nm to 100 nm. The reliability of this method is however limited, because of the problems raised by the selection of the exposure duration and the displacement velocity of the semiconductor.
The present invention relates to a highly reliable device and method for ion beam etching, enabling accurate spatial control of the etching process, a control that can be performed electronically.
The invention concerns notably a device and a method for ion beam etching enabling a one-nanometer etching accuracy.
The invention relates to such a device and such a method that can be economical and easy to implement.
To this end, the invention concerns a device for ion beam etching that enables producing an etched surface on a semiconductor or an insulant. The device comprises:
a positive ion source,
means for guiding an ion beam thereby directing the beam to the etched surface, and
means for displacing the etched surface relatively to the ion beam.
According to the invention, the device comprises:
a system for space-time detection of ion interactions of the beam with the etched surface,
means for interrupting the ion beam, and
a processing unit linked to the displacement means, to the detection system and the beam interruption means, controlling the successive operations of: detecting the interruptions of the ions in the beam with the etched surface, beam interruption, relative displacement of the etched surface in relation to the position of the beam and restoring the beam.
Preferably, the processing unit controls the operations mentioned above repeatedly.
In this context, the word ( less than  less than surface greater than  greater than  means a superficial part of the semiconductor or insulant, generally cut approximately along a crystallographic plane. The surface is advantageously plane, but can also be curved.
The operations performed with the ion beam are performed under vacuum. This vacuum may correspond to relatively high pressure, for example in the order of 10xe2x88x929 Pa. It can also be an ultra-vacuum.
The means of relative displacement of the etched surface with respect to the ion beam may involve displacement of the semiconductor or insulant or displacement of the beam, by varying their positions or their orientations. Etching is then carried out in succession, one zone after the other. According to an embodiment, several beams are directed simultaneously towards the surface.
The space-time detection system detects the positions and the timing of interactions, simultaneously.
The etching device according to the invention enables, with respect to the existing devices, reducing the exposure durations of the etched surface and the relative displacements of the etched surface with respect to the ion beam. This control improves the reliability of the system quite considerably. Indeed, the main difficulty of ion beam etching lies in the fact that the ions reach the target erratically, spatially as well as temporally. The device according to the invention enables suiting the etching process to these erratic phenomena.
The positive ions sent by the source are preferably multicharged, i.e. each of them has three positive charges at least.
Preferably, interactions of the beam ions with the etched surface are carried out one ion after the other.
The etched semiconductor is for instance formed of a material selected among Si, AsGa, InP and Ge. The etched insulant is, for its own part, formed by SiO2 or LiF.
Preferably, the etching device comprises a system for spatial localisation of the ion beam, interposed between the ion source and the etched surface.
This spatial location system consists advantageously of one or several beam collimators.
It is also advantageous that the etching device should be fitted with a system for controlling the position of the beam, and with means for cooling the beam down. It is also interesting that the device should comprise a system for monokinetic selection of the ions (between the ion source and the etched surface).
The beam interrupting means comprise advantageously means for applying an electric field that is more or less parallel to the etched surface.
This electric field is capable of diverting the beam and of preventing, thus, a new ion from reaching the surface. The use of the beam interrupting electric field is advantageously combined with that of a collimator, whereas a weak electric field is sufficient for interrupting the beam.
According to a preferred embodiment of the guiding means, the former may comprise means for applying a magnetic field, thereby diverting the ion beam by a certain angle.
Advantageously, this magnetic field is uniform and the deviation angle is 90xc2x0.
In a first variation, the guiding means comprise means for applying an electric field causing an electric deviation.
In a second variation, the guiding means implement at the same time a combined magnetic and electric field, for example in a Wien filter.
The means for relative displacement of the etched surface with respect to the ion beam comprise advantageously at least one element selected among a piezoelectric quartz and a ceramic, thereby displacing the semiconductor with respect to the ion beam incident on the etched surface.
Such displacing means enable controlled movement with accuracy in the order of one nanometer. The presence of several additional displacing means, such as two piezoelectric quartzes or two ceramics, enables moving the semiconductor or the insulant into two directions perpendicular to one another, which enables realising any etching pattern. It is then possible to create, along a rectilinear or circular line, insulating points on a semiconductor or semiconducting points on an insulant at regular intervals in order to produce digital encoding.
Preferably, displacement of the semiconductor is perpendicular to the incident ion beam.
It can also be performed with any angle with respect to the incident beam.
It is interesting that the etching device should comprise a tunnel effect and/or atomic strength microscope, performing local topographic and/or electric conductivity control of the etched surface processed.
Generally, such a microscope enables reading an etched pattern.
The etching device according to the invention is applicable to any etched surface, passivated or not, and is valid for any etching principle according to which the electrical or chemical nature or the topography of the surface is modified locally by an ion beam.
According to a first preferred embodiment of the etching device of the invention, the surface is occupied by first molecules of the semiconductor or insulant having a first chemical or topographic nature. The ion source is a source of highly charged and low energy positive ions and the etching device comprises means for applying a deceleration voltage, conferring the ions of the beam an average controlled velocity, enabling these ions, without contacting the etched surface, transforming a number of the first molecules of the surface into second molecules with a second chemical or topographic nature, whereby these ions are back-scattered.
The expression  less than  less than highly charged greater than  greater than  positive ions means ions having at least three positive charges and preferably at least fifteen positive charges. Their energy is said to be  less than  less than low greater than  greater than  with respect to that of ions obtained using a particle accelerator, whereby the latter energy is in the order of one MeV or one GeV. The low energy of the ions is thus smaller than a few tens keVs.
The deceleration voltage is applied in order to confer the ions a very low energy, close to 0 and generally smaller than a few tens eVs. Ions can be decelerated on the target, by polarising the said target or at any point of the line, by polarising the line.
An important aspect of this embodiment of the device according to the invention is that ions do not contact the surface but, on the contrary, attract to them or energise surface electrons, then run away into the opposite direction.
Interaction of the ions with the etched surface can be carried out in two ways, according notably to the nature and the disposition of the first molecules, the value of the deceleration voltage and the number charges of the ions. According to a first, preferred, interaction embodiment, the ions extract electrons from the number of these first molecules, become hollow atoms and are back-scattered. According to a second interaction embodiment, the ions cause energisation or extraction of the electrons, causing directly sputtering of fragments of the number of the first molecules.
The extraction of electrons of a semiconductor or insulant by highly charged and low energy ions is explained in the article of Jean-Pierre BRIAND presented at the Fourteenth International Conference of the Applications of Accelerators in Research and Industry, DENTON-TEXAS, 6-9 November 1996. Schematically, a highly charged and low energy ion starts interacting with the semiconductor medium or insulant at a relatively important distance from the surface, that may reach a few tens xc3x85""s. The ion attracts and captures then conduction or valence electrons that gradually occupy the Rydberg states of the ion. The ion then becomes a hollow atom, i.e. an atom having inner layers at least partially empty and external layers occupied by excited electrons. The number of electrons captured by the ion is considerably larger than its charge since a portion of these electrons is then expelled from the ion by Auger effect. The number of electrons torn away from the semiconductor by an ion is generally equal to approximately three times its charge.
Close to the surface, the ion generates an electric image (of negative charge) which exerts an attraction force on the ion and thereby tends to accelerate its movement toward the surface. However, the extraction of electrons by the ion creates on semiconductors or insulants positive holes at the surface which compensate for this electric image. The hollow atom formed out of the ion can then be back-scattered without any contact above the surface, by  less than  less than trampoline effect greater than  greater than . The existence of contact or not and of penetration inside the semiconducting material depends on the initial kinematic conditions of the ion: beyond a critical velocity, the ion directed toward the surface reaches and enters the semiconducting material in spite of the formation of the positive holes. Conversely, the trampoline effect happens below this critical velocity. The value of the critical velocity depends on the extraction potential of the semiconducting material and on the initial charge of the positive ion.
Controlling the average velocity of the ions using the deceleration voltage enables producing the trampoline effect and giving the ions a controlled charge and energy.
After a certain travel, the hollow atoms often become hollow ions spontaneously, by Auger cascades. For simplification purposes, we shall designate by hollow atoms systematically the hollow atoms remaining from the atoms or having become ions again.
Transforming the first molecules into the second molecules can take different forms, according notably to the nature and the disposition of the first molecules, the value of the deceleration voltage and the number of charges of the ions.
In a first transformation embodiment, the first molecules have a first chemical nature and the second molecules have a second chemical nature. The extraction of the first molecules leads to the sputtering of certain atoms from these first molecules, which are replaced or not with other atoms or molecules, by sending or not an appropriate product.
According to a preferred embodiment of the etching device of the invention, corresponding to this first form of transformation, the change in chemical nature causes a change in electric nature. According to a first preferred embodiment, the first molecules are semiconducting and the second molecules are insulating, and consist respectively of SiH and of SiO2. In a second preferred embodiment, the first and second molecules are respectively insulating and semiconducting and consist for instance respectively of SiO2 and Si.
According to second form of transformation, the first molecules have a first topographic nature and the second molecules have a second topographic nature. Preferably, the first molecules then form a flat surface and the interaction of the ions generates the formation of peaks and holes.
Advantageously, according to a particular variation of the first embodiment with a change in electric nature, the first molecules have external links saturated by hydrogen atoms. The means for applying the deceleration voltage enable the ions of the beam to extract electrons from the number of the first molecules of the surface to make thus the number of the first molecules lose their atoms of hydrogen and to make the corresponding external links pending. The ions become hollow atoms after having extracted electrons and are back-scattered. Moreover, the etching device comprises a source of a product saturating the pending external links in order to form the second molecules, whereas these second molecules are insulating, the source sends the product toward the etched surface further to a passage of the ion beam.
The electrons extracted from the surface of the semiconductor are essentially electrons partaking of the external links of the first molecules. In their absence, the atoms of hydrogen saturating the external links are reduced to protons that are not linked to the surface any longer. The external links thus become pending.
In this embodiment of the etching device according to the invention, contrarily to the method consisting in forming blisters by ion shocks, it is not the topography of the surface, but its conductivity that is changed. Indeed, whereas the first molecules are semiconducting, the second are insulating. It is thus possible to generate insulating marks in the order of one nanometer, which enables increasing the storage of information by some 1002 or more with respect to the existing techniques. Moreover, it is easy and rapid to control locally the conductivity of the surface after etching, using a tunnel effect microscope.
In a first advantageous variation of the preferred embodiment of this first mode, the detection system comprises an instrument for measuring the photons transmitted as the electrons extracted from one electronic layer switch to another of the hollow atoms.
This instrument advantageously measures the X-rays transmitted. Indeed, the electrons captured in the hollow and non-sputtered atoms go down towards deeper layers while causing the transmission of X-rays, whereas filling the internal layers causes temporal marks spaced by a few tens of femto-seconds. These phenomena are described in the article of Jean-Pierre BRIAND quoted previously, as well as in an article of Jean-Pierre BRIAND et Coll. Published in Images de la Physique, 1992, pages 58-62.
The photons measured by the instrument may also consist of ultraviolets, visible light or infrared.
In a second advantageous variation of the preferred embodiment of this first mode, the detection system detects electrons transmitted by Auger effect by the hollow atoms.
In a preferred variation of the first embodiment, the detection system comprises a detection surface detecting the properties of the particles bumping against the detection surface and the back-scattered ions or hollow atoms are directed by the guiding means towards the detection surface.
In this detection variation based upon the detection of the ions or hollow atoms, a signal is produced after a flight duration, for example of one microsecond, between the interaction of an ion with the etched surface and the detection of this ion or of the corresponding hollow atom.
The properties detected consist preferably of the position, the velocity and the charge of the back-scattered hollow atoms.
This mode of detection is particularly advantageous since a signal is produced systematically at each interaction, whereas the ion or the hollow atom back-scattered having a trajectory defined accordingly and easily detectable.
The following embodiments can apply to an etched surface of the type of that of the previous method, occupied for instance by a compound of the semiconductor and hydrogen, but are also applicable to other types of surfaces, notably formed of lamellar compounds.
In a second embodiment of the etching device of the invention, the detection system detects ionised fragments of molecules of the etched surface, sputtered under the effect of interactions. It may be for example silicon atoms disconnected from the target at a distance.
When the first and the second embodiments are combined, the detection system detects ionised fragments of the number of the first molecules. These fragments consist advantageously, for the preferred embodiment, of cores of hydrogen atoms lost by the number of the first molecules.
These protons transmitted and possibly re-accelerated during the separation of hydrogen from the surface can sign an impact temporarily.
In a third embodiment of the etching device, the detection system detects a burst of electrons transmitted under the effect of interactions.
In a fourth embodiment of the etching device, the detection device comprises an instrument for measuring the photons transmitted by atoms from the etched surface.
In a fifth embodiment of the etching device, the detection system detects an electric charge generated in the semiconductor by the interactions.
The detection system may implement simultaneously several detection techniques in order to obtain additional pieces of information or to bear them out.
The invention also relates to an ion beam etching method enabling to produce an etched surface on a semiconductor or insulant. In this method:
positive ions are produced,
a beam of these ions is sent towards guiding means,
the ion beam is directed to the etched surface using the guiding means,
the etched surface is moved relatively to the ion beam.
According to the invention, the following operations are performed iteratively in order to etch the surface considered:
interactions of ions of the beam with the etched surface are detected spatially and temporally,
the ion beam is interrupted,
the etched surface is moved relatively to the position of the beam, and
the ion beam is restored.